How Condensing Boilers Work

Many homeowners looking to save money on heating costs this year will have come across Condensing Boilers. Condensing boilers have much lower fuel running costs and higher efficiencies than standard efficiency boilers otherwise known as ‘Conventional’ boilers.

Below we explain how condensing boilers work and what their advantages are. We even have an on-site video so you can view one that has been installed.

Condensing boilers are now the main replacement for standard efficiency boilers. The UK govenment have made it law that all gas and oil boiler replacements must be high efficiency condensing boilers – only making allowances for certain circumstances.

Why buy a condensing boiler?

The main reason to buy a condensing boiler is the large saving in fuel and therefore money. Most standard efficiency boilers will operate at 70%-80% efficiency (some older boilers as low as 50%) A condensing boiler will run at an optimum efficiency of 95% sometimes rising to 98% in certain conditions. The reduced boiler operation time reduces your carbon emisions.

How does a condensing boiler acheive such high efficiency?

The first thing to remember about conventional boilers and condensing boilers is the heat input from the burner is the same. Roughly 250-300 degrees celsius.

It is important to understand how a conventional boiler works in order to compare the two.

A conventional boiler has one heat exchanger. Water enters the heat exchanger at the coldest (and generally lowest) point and collects heat leaving the heat exchanger and flowing off to the radiators and hot water tanks. The flue temperature can be anything up to 250 degrees celsius. That heat is expelled into atmosphere never to be used again. Wasted.

A condensing boiler has two main heat exchangers. This time the water enters the secondary (and condensing) chamber first. Picking up latent heat from the hot flue gases as they cool. The moisture in the gases condenses into droplets that form at the bottom of the heat exchanger. The condense is mildly acidic (about the same pH as tomato juice) and is expelled to drain. From there the water enters the primary heat exchanger to collect more heat before flowing to the radiators and hot water tanks. The flue temperature will be around 55 degrees celsius. That means up to 200 degrees of heat is being transferred to your heating water, not expelled into atmosphere. This means you save loads of money.

Condensing boilers do not condense all the time – only when the return water temperature is low enough. However you are always saving money because of the greater heat exchanging surface area offered by an extra heat exchanger anf the improvements in modern design.

Myths about condensing boilers and answers


1. They cannot be installed to an existing system. This is not true. A condensing boiler can be fitted to any system that previously had a conventional boiler installed. Sometimes the increased efficiency means a feed and expansion system is not suitable. This is easily overcome however by pressurising the system (many engineers will do this as a matter of course)

2. They are not as reliable. Again this is not true. In their infancy boiler manufacturers did have problems dealing with the slightly acidic condensate attacking the boiler heat exchanger but like all things they have improved to the piont that this is no longer a problem. All components are now just as reliable as conventional boilers.

3. They are only more efficient when fully condensing. Not true. All condensing boilers are more efficient then conventional boilers at all times of operation. The British Research Establishment confirmed this in a paper published in 2003 after research going back to 1980.

4. The condense is a problem. The condense can be easily installed by running to a safe drain or lime chipping soakaway to nuetralise the pH. Manufacturers even produce an inline acidic nuetraliser that has puch fit fittings for easy installation to 3/4″ condense pipe.

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